1. Field of the Invention
This invention relates to a valve actuator and, more specifically, to such a valve actuator which normally operates with hydraulic oil being utilized to open a valve by acting on a piston in opposition to high pressure gas of an accumulator which provides a closing force when the hydraulic oil is discharged and alternatively allows hydraulic oil to be used for backup closure of the valve.
2. Description of the Prior Art
There have heretofore been utilized a number of means for closing large valves of the type found in feedwater and steam systems of nuclear power plants. It has been consistently felt advantageous for such valves to be capable of rapid closure. Because of safety and reliability requirements of these plants, the time required to close the valves within the system and the ability of the valve to safely withstand seismic shock are of real concern. One valve which has been found to fit these requirements includes a hydraulic cylinder mounted on the valve in alignment with the valve stem. A piston means slidably disposed within the cylinder includes a rod extending from a first side thereof through a first end of the cylinder to be joined to the valve stem. A high pressure gas accumulator is mounted about the second end of the cylinder to allow communication of the accumulator with the second side of the piston. The accumulator in the preferred embodiment of this device is generally spherical in shape, which shape has been found to minimize the overall weight of the actuator to reduce its cost and to lessen any seismic problems which might exist.
During normal operation, hydraulic oil is supplied to the cylinder to act on the first side of the piston causing it to move in opposition to the high pressure gas. Discharging the hydraulic oil then allows high pressure gas to act on the piston which, in turn, would close the valve.
While this device has been considered an effective, reliable means for valve operation, there has been expressed some concern as to a means for closing the valve should a rupture of the spherical accumulator or extensive leakage of the high pressure gas therefrom occur to interfere with the ability of the valve actuator to close the valve if required.